Since the development and widespread implementation of Erbium-doped fiber amplifiers (EDFAs) the development of optical fiber based communication systems has been a balance between various factors. The additive Gaussian noise induced by EDFAs drives systems to use higher launched powers to increase the signal to noise ratio (SNR) so that low bit error rates (BERs) can be maintained. However, because of non-linear effects in optical fiber, when the launched power is too great impairments are induced that can severely limit performance. Examples of such impairments are self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM). Therefore, for each system there will be some optimal launched power that maximizes SNR without introducing limiting non-linear penalties.
Dispersion is a physical property of optical fibers that can induce system penalties, and is most difficult to manage for high local dispersion fibers, long reach systems, and high bit rate systems. In the absence of non-linear impairments, the ideal fiber dispersion would be zero. However, this greatly enhances FWM. Therefore, another balancing to be managed is the need to have overall low accumulated dispersion in a system with sufficiently high local dispersion to manage FWM and other non-linear penalties. Appropriate choice of a dispersion map in the system can reduce non-linear penalties and can enable use of higher launched powers.
Dispersion units are typically given as picoseconds/nanometer-kilometer (ps/nm-km), where the kilometer units correspond to the length of the fiber. The dispersion product of a span of fiber is a measure of the dispersion accumulated over the span. The dispersion product for a fiber of length L with a dispersion D is the product of L and D, i.e., L·D. Thus, the dispersion product of a span of fiber having individual section of length Li and dispersion Di is the sum of the individual dispersion products ΣLi ·Di at a given wavelength.
Nonlinear optical effects (such as four-wave mixing (FWM) and Cross-Phase Modulation (XPM)) can degrade the optical signal transmission through long-haul optical networks. Increasing the dispersion in the fibers decreases both FWM and XPM. Dispersion causes broadening in transmitted optical pulses due to the difference in transmission speeds of light at different wavelengths. Because the group velocity difference between channels is proportional to dispersion, a larger group velocity difference between channels implies that one channel walks over the other channel at a very fast rate and collision length is very short. If collision occurs very rapidly, the impact of the collision is minimal. Therefore, it is advantageous to have large dispersion so that collision length is short.
While dispersion reduces nonlinear effects such as FWM and XPM, the accumulated dispersion in these long-haul systems must be compensated. In long-haul repeatered transmission systems using optical fibers, the interplay of the accumulation of large amounts of the chromatic dispersion and self-phase modulation (SPM), creates noise and distortion in the optical system. Dispersion maps, i.e., the dispersion as a function of the transmission distance, attempt to minimize the effects of chromatic dispersion.
Another balancing to be managed in optically transparent networks is the choice between the optimal dispersion map to suppress non-linear penalties and the best map to facilitate all-optical networks. For high bit rate (>40 Gb/s) RZ systems, non-linear impairments can be reduced when the local dispersion is very large and the accumulated dispersion before compensation is also large. However, such dispersion maps may be difficult to manage in an optically transparent network, for example because signals arriving at a switching node may have significantly different accumulated dispersion values, inhibiting the ease and accuracy of optical performance monitoring (OPM).
One known attempt at balancing these considerations is to use dispersion managed fiber or cable which can provide relatively low-loss spans, large local dispersion and zero accumulated dispersion on a span-by-span basis. However, certain non-linear penalties—inter-channel XPM (xXPM) and intra-channel XPM (sXPM)—are reasonably enhanced when each span has zero accumulated residual dispersion. Another known technique proposed in the literature uses an optical phase conjugator, which inverts the phase induced by the non-linear interactions so that propagation forward from the location of the phase conjugator will undo the non-linear phase, but the technique does not utilize dispersion symmetry about the point of phase conjugation.